Sudden Rashes On Skin, In Arm Pits, Behind Neck, Itching, Scratching. Causes?

i developed a rash that comes out where i tend to swear the most. under my arm pit and the back of my neck . It spreads if i scratch it and it is kind of itchy, but it is not too itchy to the point here i feel the need to do scratch. It started out as just red dots under my skin, but became raised red blotches. Then after a a few days of not scratching and applying lotion, it turn brown. Then my side/back started itching and i scratched it and the rash appeared there all along my back. I take showers everyday, but don t know what this rash is or how i got it. Why did the red rash turn brown? I have also not been using any new skin products, it just appeared, but the weather where i live did get hit with a heat wave for a month.